To evaluate ZDV+3TC vs D4T+ritonavir vs. ZDV+3TC+ritonovir with respect to the change in plasma HIV-1 rna copy number from baseline to 48 weeks in stable HIV-infected children with greater or equal to 16 weeks of prior continuous antiretroviral therapy and to evaluate the safety adn tolerance of ZDV+3TC vs. d4T+ritonavir vs. ZDV+3TC+ritonovir based upon laboratory and clinical toxicities